This core provides input from what is generally agreed to be the leading EPR instrumental development site in the world, the National Biomedical EPR Center at the Medical College of Wisconsin (MCW). The role of this core will be to provide leadership in the development of specific aspects of the technology needed to accomplish the goals ofthe CMCR. These developments will be carried out in collaboration with the projects and with the instrumental Core at Dartmouth, to facilitate the development of the best possible o prototype instruments for EPR dosimetry. The contributions ofthe core at MCW are described in four specific aims. Specific Aim 1 (50% of effort) is the Development of resonators for measurements at X-Band in vivo for nails and teeth; this will be the most extensive project and will focus on exploiting the breakthrough achieved in initial collaborative studies where for the first time, resonators that operate at the very sensitive X-Band frequency were successfully used to make measurements in vivo. This development is at the heart of project 3. During the course of the grant this core will attempt to make analogous resonators that can be used for in vivo tooth dosimetry, which could dramatically improve the already impressive sensitivity of this approach (Project 1). Specific Aim 2 (25% of effort) - Development of instrumentation to support measurements at X-Band, will especially focus on development of specifically designed microwave bridges that will facilitate both Projects 2 &3. These developments will be carried out in collaboration with a longstanding collaborative partner of both Dr. Hyde and Dr. Swartz, Dr. Froncisz of the Jagiellonian University in Krakow (Dr. Froncisz's support will come via a purchase agreement. Specific Aim 3 (20% of effort) - Improvements in resonators and bridges for measurements at L-Band. This core will similarly work with Dartmouth and Krakow in the development of improved L-Band bridges, in support of Project 1. It also will facilitate resonator development in project 1 by use ofthe very sophisticated modeling techniques that are highly developed at MCW, which enable different configurations of resonators to e=be rigorously evaluated so that construction can be focused on designs that are likely to be successful Specific Aim 4 -(5 % of effort) support of multifrequency studies will available an exceptional set of EPR spectrometers that can be of value to all three projects in helping to elucidate the characteristics of potentially overlapping components of EPR spectra.